1. Field of the Invention
The present invention relates to a blood pressure meter cuff equipped with a fluid bag into which a fluid is injected and a fixing tool fixing the fluid bag.
2. Description of the Prior Art
There have been available various kinds of non-invasive blood pressure measuring methods, among which, currently, an oscillometric method (or an oscillation method) is most widely proliferated. The oscillometric method is conducted such that a tourniquet in a belt-like shape called a cuff having a fluid bag therein is fixedly wrapped around a limb and an internal pressure is given to the fluid bag to pressurize an artery in the limb to capture a change in arterial volume determined in a balance between an pressurization pressure as a reaction and a pressure in the artery and estimate a blood pressure value. To be more concrete, since a change in arterial volume is sensed as a change in internal pressure in the fluid bag. (which is a pulse wave) and an amplitude thereof is altered depending on a blood pressure and an pressurization pressure, a blood pressure can be estimated from a change in pulse wave amplitude relative to a pressurization pressure.
While the oscillometric method was developed originally as a method using an arm as a measuring site, commercialization on a blood pressure meter in recent years has proceeded to provide new types measuring a blood pressure at sites other than an arm, such as a wrist and a finger. The new types of blood pressure meters have been increasing a need for themselves on the market because of advantages such as no necessity for taking off clothes in mounting a cuff, which is necessary for an arm type, and a small size and high portability.
Since, especially, a wrist blood pressure meter uses an artery located near an arm as a measuring site, usefulness is generally accepted to be higher than a finger type, though with a convenience at the same level as the wrist type. A cuff of a wrist blood pressure meter is generally mounted at a site in the forearm side adjacent to a wrist joint. According to Alexander H. et al. (see non-patent literature 1), a theory is such that in order to properly pressurize a blood vessel of a limb for measuring a blood pressure, it is required that a width, in an axial direction of a limb, of a fluid bag attached to a cuff in use, that is a cuff width is of 40% or more of a diameter of the limb at a measuring site and blood pressure meters currently on the market are designed according to this theory. Therefore, in a case of a wrist blood pressure meter put into practice, a cuff width is on the order in the range of from 50 to 60 mm.
In actual cases of adopting this kind of cuff, a problem has been arisen to some of users that a pressurization pressure of a fluid bag does not correctly act on a target artery and the target artery is not sufficiently pressurized, therefore disabling a blood pressure to be correctly measured. There are two arteries called the radial artery and the ulnar artery in a wrist and the two arteries each are located in a slightly retreated position surrounded with the two bones called the radius and the ulna and tendons extending in parallel thereto. Hence, a pressurization force of a fluid bag is harder to reach a target artery in the cases than the brachial artery that is an target having been conventionally measured and present in a state where no tendon exists around a bone.
It has been conventionally known that a cuff pressure is not sufficiently transmitted to a blood vessel, in the axial direction of a limb, near both ends of a fluid bag of a cuff of a blood pressure meter to thereby disable the blood vessel to be sufficiently pressurized. This phenomenon is called as an edge effect. If this edge effect occurs, pulsation occurs in a blood pressure in a blood vessel not sufficiently pressurized near an end of a cuff fluid bag. In this case, this pulsation is sensed as a change in cuff pressure, leading to a factor for measurement error if a chance allows.
In order to cancel this problem, a cuff has been conventionally known that pressurizes a wrist joint section easy to be pressurized because an artery is located in the downward vicinity of a body surface. Since, in the wrist joint section, a blood vessel runs outside of the condyle, the blood vessel is located at a shallower position below the skin than a blood vessel in the forearm side, located between the radius and the ulna, the vessel running outside of the condyle is easier to be pressurized. A prominence called the radial (or ulnar) styloid process 32 is present at the peripheral end of the radius (or the ulna) 33, in which portion, especially, a blood vessel 31 is conspicuously present in the downward vicinity of the body surface (see FIG. 6).
The non-patent literature is Alexander H. “Criteria in the choice of an occluding cuff for the indirect measurement of blood pressure,” in Medical & Biological Engineering & Computing, (U.K.) issued by Institution of Electrical Engineers, 1977, Vol. 15, pp. 2 to 10).
According to observation by the inventors, it has been found that in a case where a pressure is applied with a cuff having a conventional width on a portion including a site on the body surface in the downward vicinity of which the blood vessel exists, a good result is not necessarily obtained. To be concrete, a phenomenon has been experienced by some of patients that deformation, which is encountered when a blood vessel is not sufficiently pressurized, occurs on a curve of a change in pulse wave amplitude changing depending on a cuff pressure.
Description will be given of a cause for this phenomenon with reference to FIG. 6. It is only around the radial styloid process 32, for example, in a case of the radial artery 31, that a blood vessel is conspicuously present in the downward vicinity of the body surface near the wrist joint and a length of this section is roughly on the order in the range of from 15 to 25 mm. This section is indicated as a section B in FIG. 6. Since a cuff width for a conventional blood pressure meter is, as described above, in the range of from 50 to 60 mm and therefore, a blood pressure 31 is pressurized additionally in sections in the forearm side and the peripheral side adjacent to the section B (the sections A and C of FIG. 6), a change in arterial volume, which is a reaction of the pressurization, that is a pulse wave is also sensed from the blood vessel 31 in the additional sections.
Since the blood vessel 31 is, however, located clearly in deeper positions in the sections A and C than in the section B and surrounded with bones and tendons, the vessel 31 is harder to be pressurized. Therefore, the blood vessel 31 is not occluded even under a pressure which is higher than a blood pressure and so high that the blood vessel is to be occluded by nature. It is thought that the blood vessel. 31 in the sections A and C is in a state lower in cuff pressure than in the section B, that is in a state where a change in arterial volume is not extinct, in other words in a state where the blood vessel 31 is pressurized not to be flat, again in other words, in a state where the edge effect occurs. This hypothesis has been confirmed to be true as a result after the inventors conducted experiments using a transparent cuff and observed pulsation at the upstream and downstream ends of the cuff under a high cuff pressure.
The reason why, though the blood vessel is perfectly occluded at the center of the cuff and a blood flow is blocked, pulsation occurs not only at the upstream end of the cuff, but also at the downstream end thereof, is that the radial artery and the ulnar artery communicate with each other through a thick blood vessel in the palm section more down stream and in a case of a cuff in which only the radial artery is selectively pressurized, a blood flow from the ulnar artery is fed to the cuff down stream end of the radial artery.
If a blood vessel located in the downward vicinity of the body surface is pressurized by a cuff having a fluid bag designed dimensionally in consideration of a relation with a wrist circumference according to the conventional Alexander H. et al.'s theory, an influence from a blood vessel at a site other than a blood vessel in the downward vicinity of the body surface is exerted, leading to a problem to cause a result of an incorrect blood pressure measurement.